Rotating equipment, especially gas turbines, steam turbines or compressors are often evaluated by their power output or intake, efficiency and production costs. In case of a gas turbine the ratio of the price to the power output is an important economic indicator for the customer. Considering the average lifetime and the fuel prices forecasted for the life time span simple mathematics allow first estimations, which assists a buyer's decision. From the supplier's perspective low production costs and high efficiency contravene each other considering most of the technical solutions to enhance efficiency.
One possibility of efficiency enhancement is proposed in DE 39 41 174 A1. The teaching of this disclosure is that a beneficial effect on efficiency can be achieved by a reduction of radial rotor clearance, which can be realised by cooling an inner casing, which is mounted in an outer casing by means of cool air, bled off from the compressor of the gas turbine. The cooling of the inner casing reduces the thermal expansion of this part and this shrinking reduces the radial clearance between stationary seals and rotating blades respectively stationary blades and rotating seals. Hence losses caused by secondary flows in the hot gas path of the gas turbine are reduced and the efficiency respectively the total power output can be increased.
However such a solution is expensive. Depending on the prices for fuel cheaper machines with a lower efficiency can better be put onto the market.